1. Field of the Invention
This invention relates to a photosensitive medium usable with the wet electrophotographic system and having a photoconductive layer resistant to liquid developer. Generally, the configuration of the photosensitive medium for electrophotography is determined by the type of electrophotography with which it is used. The electrophotographic system may generally be classified into two types, one of which provides electric charge directly on the photoconductive layer and the other type provides electric charge on a dielectric layer formed on the photoconductive layer. The latter system is more advantageous in respect of the characteristic of the photosensitive medium itself and the higher electrostatic contrast provided to form a clear copy image. The photosensitive medium of the present invention is directed for use with the said latter type of system.
2. Description of the Prior Art
The electrophotographic system of the described type using such photosensitive medium has been proposed, for example, by Japanese Pat. No. 23910/1967 and Japanese Pat. No. 21748/1968, each of which discloses a method comprising the steps of charging a photosensitive medium through corona discharge in light or dark regions to thereby cause the charge to be trapped in the vicinity of the interface between the photoconductive layer and the transparent dielectric layer of the photosensitive medium, thereafter projecting image light upon the photosensitive medium while applying a charge of the opposite polarity or AC corona discharge thereto to reverse or erase the charge in the light regions of the photosensitive layer by the utilization of the impedance difference between the light and dark regions of the photoconductive layer, thereby forming a latent image having a contrast of electrostatic potential, then developing and transferring such latent image through conventional processes to provide a copy image.
In the past, the photosensitive medium has been used with the dry development to form images, but if used with the wet development, it would probably bring about various advantages such as speed-up of the development and hence of the copying, improved definition of the resultant images due to the fine toner particles, simplified construction of the developing device and accordingly reduced cost of the copying apparatus. Nevertheless, this has not been put into practice mostly for the reason that the characteristic of the photoconductive layer is deteriorated by the dispersion liquid contained in the liquid developer. For example, in case of a photosensitive medium having a photoconductive layer formed of non-crystalline Se or Se-Te alloy, the cross-sectional portion of such photoconductive layer which is exposed may be affected by its contact with the dispersion liquid in the liquid developer, such as kerosene, silicone oil, carbon tetrachloride or the like, or a mixture thereof of various organic solvents slightly present therein, so that crystallization will gradually progress in the photoconductive layer until its photoconductivity is entirely lost. This is particularly so in the copying machines or the like where both the photosensitive medium and the developing liquid often experience the temperatures of 40.degree. to 50.degree. C.
Also, in case of a photosensitive medium whose photoconductive layer is formed of photoconductive fine particles dispersed in a resin, the formation of such photoconductive layer is done by applying and drying a paint of photoconductive material and resin binder: in the process of drying the solvent, the photoconductive layer unavoidably becomes porous since it is desirable to reduce the percentage of the binder resin in the photoconductive layer as much as possible in order to enhance the sensitivity of the layer. Thus, the dispersion liquid contained in the developer would generally intrude into the photoconductive layer through the cross-section thereof to thereby reduce the resistance of the photoconductive layer to the dark, which in turn leads to an insufficient potential in the dark regions of a copy image and accordingly to a lower contrast of the final copy image. Especially, when organic solvents are present mixed in the dispersion liquid, the binder resin would dissolve to destroy the photoconductive layer and the effluent binder resin would variously affect the developing liquid.
This is also the case with a photoconductive layer which comprises a coating of polyvinyl carbazole and other various organic photoconductive materials or a coating of organic photoconductive material and resin.